Ink-jet recording has been used in a wider range of applications than before; it is used at home, at office, for photography, and out of doors. Printed matters therefore need to have a weather resistance (light resistance) and a storability. In ink-jet recording, dyes have been mainly used as colorants; however, they have problems in terms of light resistance, and thus aqueous ink-jet recording inks in which pigments having a good light resistance are used have been developed.
Since pigments are water-insoluble, techniques for stably dispersing them in aqueous media are necessary. In a known one of such techniques, a pigment is coated with a self-water-dispersible resin in which some of acid groups have been neutralized with basic groups, such as a styrene-(meth)acrylic-acid-based copolymer in which some of (meth)acrylic acid groups have been neutralized; and this pigment is dispersed and diluted with an aqueous medium (for instance, see Patent Literature 1). Such a pigment dispersion using the resin dispersant, however, generates coarse particles in some cases.
Coarse particles are particles having a particle size so much larger than the average particle size of an obtained aqueous pigment dispersion, and they are speculated to be formed by non-dispersion of a pigment, agglomeration of a pigment that has been dispersed but broken, or agglomeration of a polymer to be used as the pigment dispersant.
Such coarse particles prevent formation of the even surface of a coating film in painting and printing and cause nozzle clogging of an ink-jet head in printing in which an ink, such as an aqueous ink-jet recording ink, is ejected from the nozzle. In particular, as ink-jet printers have become more high-resolution these days, the nozzles of an ink-jet head are arranged at higher density, and finer liquid droplets are ejected, in other words, the nozzles for ejecting ink each have a smaller diameter and are more integrated (for example, see Patent Literature 2). As the diameter of nozzles becomes smaller, the acceptable size of foreign objects becomes smaller; as a result, nozzle clogging is more likely to be caused, and thus inks that have been able to be used in typical ink-jet printers cannot be used in a new printer that has been developed to have a high-resolution ink-jet head, which is problematic. Accordingly, a reduction in coarse particles is necessary in applications to high-resolution ink-jet printers.
In a known technique for reducing generation of coarse particles having a large particle size, for example, a block polymer compound having a hydrophobic segment and a hydrophilic segment including a hydrophobic unit and hydrophilic unit is used as a pigment dispersant (for instance, see Patent Literature 3). In the block polymer compound disclosed in Patent Literature 3, the hydrophobic segment, which consists of the hydrophobic unit, as a repeating unit and the hydrophobic unit and hydrophilic unit as a repeating unit form a random copolymer structure or a gradient structure; the hydrophobic segment is oriented to be inside in an aqueous solvent, so that a micelle in which a hydrophilic shell is disposed around a hydrophobic core is formed; in the case where a liquid composition in which the block polymer compound having such a structure is dispersed is applied to a recording medium by an ink-jet technique and where the surface of the recording medium is observed with, for example, an electron microscope, coarse particles are not found; and in the case where a liquid composition in which a block polymer compound having a hydrophobic segment and a hydrophilic segment consisting of only a hydrophilic unit is dispersed is used, spherical coarse particles having a diameter of approximately 100 nm are found.
Coarse particles are, however, generated even with the technique disclosed in Patent Literature 3 in some cases. In addition, Patent Literature 3 does not refer to a reduction in the absolute number of coarse particles at all.